An ancient, bipedal hominid sporting a set of powerful jaws and huge molars that earned it the nickname "Nutcracker Man" likely didn't crack nuts at all, preferring instead to slurp up vast quantities of grasses and sedges, says a new study.

The hominid, known as Paranthropus boisei, ranged across the African landscape more than 1 million years ago and lived side-by-side with direct ancestors of humans, said University of Colorado Boulder anthropology Professor Matt Sponheimer, a study co-author. It was long assumed Paranthropus boisei favored nuts, seeds and hard fruit because of its huge jaws, powerful jaw muscles and the biggest and flattest molars of any known hominid in the anthropological record, he said.

In the last several years, research on the wear marks of teeth from Paranthropus boisei by other research teams has indicated it likely was eating items like soft fruit and grasses, said Sponheimer. That evidence, combined with the new study that measured the carbon isotopes embedded in fossil teeth to infer diet, indicates the rugged jaw and large, flat tooth structure may have been just the ticket for Paranthropus boisei to mow down and swallow huge amounts of grasses or sedges at a single sitting, he said.

"Frankly, we didn't expect to find the primate equivalent of a cow dangling from a remote twig of our family tree," said Sponheimer.

Published in the May 2 issue of the Proceedings of the National Academy of Sciences, the study was led by University of Utah Professor Thure Cerling. Other authors included Emma Mbua, Frances Kirera, Fredrick Manthi and Meave Leakey from the National Museums of Kenya, Fredrick Grine from Stony Brook University in New York and Kevin Uno from the University of Utah.

"Fortunately for us, the work of several research groups over the last several years has begun to soften prevailing notions of early hominid diets," said Sponheimer. "If we had presented our new results at a scientific meeting 20 years ago, we would have been laughed out of the room."

For the new study, the researchers removed tiny amounts of enamel from 22 Paranthropus boisei teeth collected in central and northern Kenya, each of which contained carbon isotopes absorbed from the types of food eaten during the lifetime of each individual. In tropical environments, virtually all trees and bushes -- including fruits and leaves -- use the so-called C3 photosynthetic pathway to convert sunlight into energy, while savannah grasses and some sedges use the C4 photosynthetic pathway.

The isotope analysis indicated Paranthropus boisei individuals were much bigger fans of C4 grasses and sedges than C3 trees, shrubs and bushes. The results indicated the collective diet of the 22 individuals averaged about 77 percent grasses and sedges for a period lasting at least 500,000 years, said Sponheimer.

The research team also compared the carbon isotope ratios of Paranthropus teeth with the teeth of other grazing mammals living at the same time and in the same area, including ancestral zebras, hippos, warthogs and pigs. The results indicated those mammals were eating primarily C4 grasses, virtually identical to Paranthropus boisei. "They were eating at the same table," said Cerling.

Paranthropus was part of a line of close human relatives known as australopithecines that includes the famous 3-million-year-old Ethiopian fossil Lucy, seen by some as the matriarch of modern humans. Roughly 2.5 million years ago, the australopithecines are thought to have split into the genus Homo -- which produced modern Homo sapiens -- and the genus Paranthropus, that dead-ended, said Sponheimer.

"One key result is that this hominid had a diet fundamentally different from that of all living apes, and, by extension, favored very different environments," he said. "And having a good idea of where these ancient creatures lived and what they ate helps us understand why some early hominids left descendants and others did not."

The first skull of a Paranthropus boisei individual was discovered by co-author Meave Leakey's in-laws, Mary and Louis Leakey, in 1959 in Tanzania.

In 2006, a team led by Sponheimer found that a cousin of Paranthropus boisei known as Paranthropus robustus had a far more diverse diet than once believed, clouding the notion that it was driven to extinction by its picky eating habits. Published in Science magazine, the study showed that Paranthropus robustus had a diverse diet ranging from fruits and nuts to sedges, grasses, seeds and perhaps even animals.

So what led to the end of the line for Paranthropus? It could well have been direct competition with Homo -- which was becoming skilled in extensive bone and stone technology -- or it could have been a variety of other issues, including a slower reproductive rate for Paranthropus than for Homo, he said.

The new study was funded by the National Science Foundation and the CU-Boulder Dean's Fund for Excellence.

Paranthropus boisei, an ancient human relative that was nicknamed "Nutcracker Man" because of its powerful jaws and large, flat teeth, did not eat nuts but instead dined primarily on grasses, according to a new study involving CU-Boulder. Skull photo courtesy of the National Museums of Kenya.